As you read what follows, remember that heat energy naturally flows from a hot environment to a cold one until the temperatures are equal.

But we can use heat pumps to make heat go where we want — refrigerators and air conditioners are examples of heat pumps. There are now devices simply named "heat pumps," avoiding all pretense, whose task is to heat or cool a house or environment. All such devices take advantage of the relationship between pressure and temperature in a gas or liquid.

A typical heat pump uses electrical energy to drive a compressor. The compressor increases the pressure in one vessel and reduces it in another, in order to move (or "pump") heat from one place to another.